Microbial fuel cells were developed primarily for the conversion of waste products (sewage, farming wastes, etc.) into electrical energy. However, other applications of microbial fuels cells include use as analytical sensors and bioremediation. The primary difference in the microbial fuel cells for energy production and/or bioremediation applications versus the analytical applications is the magnitude of the current generated. Energy production and bioremediation application generally require anodes and cathodes with large surfaces to increase the production of current, whereas high current is not of primary importance for analytical sensor applications.
Microbial fuel cell technology can be roughly divided into two basic types of designs: 1) reactor designs, and 2) probe designs. The two designs have been used for energy production, bioremediation, and analytical applications. The differences between the two designs are based on: 1) the placement and orientation of the anode and cathode, 2) method of substrate (oxidizable organic materials) introduction to the anode, and 3) the method of providing the ultimate electron acceptor (e.g., oxygen, ferricyanide, and the like) to the cathode.
Reactor Designs
A reactor design typically incorporates the anode inside of an anode chamber where the wastewater or natural waters are flowed through the chamber. The cathode design is considerably more varied and includes: 1) air cathodes, 2) cathodes located in a separate cathode chamber, and 3) poised electrodes. Air cathodes are incorporated into the wall of the anode chamber and allow the diffusion of atmospheric oxygen through a semi-permeable membrane to a cathode located within the anode chamber. Cathodes located in a separate cathode chamber are separated from the anode chamber using a semi-permeable membrane. The cathode chamber is typically filled with an oxygenated aqueous solution, or alternatively filled with a solution (e.g., ferricyanide) capable of accepting electrons from the anode. The chamber design is used for energy production, bioremediation, and analytical applications.
The reactor design applications for analytical sensor have been primarily limited to applications where high concentrations (e.g., millimolar (mM)) of organic substrates exist in wastewaters and/or sludges associated with sewage plants and solid waste disposal facilities. One significant application for microbial sensors is the determination of biological oxygen demand (BOD) in wastewaters. Microbial sensors designed for the determination BOD of wastewaters typically use a reactor design. Wastewater is transported in pipes at treatment facilities, and therefore it is convenient to divert the flow of the wastewaters through reactors for the measurement of BOD.
Typical BOD sensors measure the electrical current between the anode and cathode (or poised electrode) as the metric for BOD. Disadvantages of such sensors using a reactor design for analytical applications for characterizing submerged sediments and natural waters include:                The reactor design requires the substrate in water to be passed through an anode chamber. This is not a viable option if the anode is being directly inserted in sediments, soils and groundwater.        Reconfiguration of reactor designs to match the actual site conditions is difficult and not suitable for a majority of the sites.        Most reactor designs are optimized (anode and cathode size, microbial composition, and performance) for energy production that is not an important parameter for an analytical sensor.        Reactor design is not convenient for the deployment of multiple sensors to characterize the chemical (oxidizing and reducing) environment of a site.        
The sensors rely on current measurement to determine microbial activity/substrate concentration; such current measurement may not be sensitive enough to measure desired microbial activity.
Probe Designs
Probe design generally include separate anode and cathode components that are not placed into chambers. The probes are usually placed into natural environments, or artificial ponds/digesters at wastewater treatment facilities. The anode is placed in either anaerobic sediment in natural environments, or at an anaerobic zone in wastewater treatment ponds/digesters. The cathode is typically placed into an oxygenated zone above the anaerobic zone where the anode is deployed. The probe design is used for energy production, bioremediation, or analytical applications.
Anode and cathode probes are used in the production of electrical power in marine environments. In these cases, the anodic probe is buried in the anaerobic marine sediments and the cathodic probe is positioned above the anaerobic sediments in the oxygenated water. The typical application of these benthic probes is the production of energy for navigation buoys and other marine instrumentation. Benthic probes are primary used for power production and not as analytical sensors.
A probe that uses three electrodes for energy production and organic contaminant removal at wastewater or sewage treatment facilities based on changing conditions of the organic contaminants present in the water is disclosed in U.S. Pat. No. 9,299,999, issued in the name of Chang et al. The three-electrode system was developed for energy production and contaminant removal, not as an analytical sensor. A primary concern of the '999 Patent is the optimization of electrical current in changing environments. The three-electrode system has a floating cathode and an anode placed into the sediment or sludge at the bottom of a digester. The third electrode is located in the water column between the anode and cathode to serve as either and anode or cathode depending on the water conditions.
A BOD analytical system combines the anode and the electron acceptor into the same probe is disclosed in U.S. Pat. No. 6,113,762, issued in the name of Kruber et al. The probe design does not use oxygen as the ultimate electron acceptor, but rather uses a three-electrode system: counter electrode, microbial electrode and reference electrode with a potentiostat.
Microbial fuel sensors have been deployed in the environment to measure microbial activity in groundwater for bioremediation applications. One application deployed an anodic probe within a monitoring well to determine the reduction of uranium (VI) to uranium (IV) at a site located in Rifle, Colo. The cathode was located at the surface of the site. Reagents were injected into the contaminated groundwater to induce the reduction of uranium. The injection of reagents resulted in relatively high substrate concentrations (e.g., on the order of mM) in the aquifer. The electrical current was measured between the anode and cathode as the metric for acetate concentrations. The cathode was placed into an oxidizing environment at ground surface.
A microbial sensor system was used to evaluate the operating characteristics of the system when exposed to very low concentrations (e.g., on the order of micromolar (uM) or nanomolar (nM)) of substrates. The results of the investigation indicated that microbial sensors have environmental applications at low substrate concentrations and/or in the evaluation of turnover rates. The electrical current was measured between the anode and cathode of the system as the metric for substrate concentration. The investigation performed in 2014 and was cited by the authors as being the first investigation of microbial sensors being exposed to very low concentrations of substrates. The anode was placed into the anaerobic zone of the chamber (bottom) while the top zone of the chamber was oxygenated. The cathode was placed into the oxygenated zone. The system was developed to determine if microbial sensors could detect current at very low concentrations in a variety of sediments, not as a practical analytical system that could be deployed in the field.
Below are illustrative disadvantages of the prior art probe designs and methods for analytical applications of characterizing submerged sediments and natural waters.                The probe designs place the anode in the anaerobic zone (sediment or water) but most designs place the cathode in an aerobic zone located above the anaerobic zone within the same pond or test chamber, or at the surface. This limits the ability to deploy the system at sites with completely anaerobic conditions.        The probes designs do allow for reconfiguration of the probes for deployment in a variety of environments including soils, sediments and groundwater.        The systems use measurement of current as the metric of substrate concentration or turn-over rate.        Multiple sensor deployments require using multiple reference electrodes.        The systems require use of a cathode or a poised electrode.        